Photovoltaic cells convert light into electric current. One of the most important features of a photovoltaic cell is its efficiency in converting light energy into electrical energy. Although photovoltaic cells can be fabricated from a variety of semiconductor materials, silicon is generally used because it is readily available at reasonable cost, and because it has a suitable balance of electrical, physical, and chemical properties for use in fabricating photovoltaic cells.
In a known procedure for the manufacture of photovoltaic cells, silicon feedstock is doped with a dopant having either a positive or negative conductivity type, melted, and then crystallized by either pulling crystallized silicon out of a melt zone into ingots of monocrystalline silicon (via the Czochralski (CZ) or float zone (FZ) methods), or cast into blocks or “bricks” of multi-crystalline silicon or polycrystalline silicon, depending on the grain size of the individual silicon grains. As used herein, the term “monocrystalline silicon” refers to a body of single crystal silicon, having one consistent crystal orientation throughout. Further, conventional multi-crystalline silicon refers to crystalline silicon having centimeter scale grain size distribution, with multiple randomly oriented crystals located within a body of multi-crystalline silicon. As used herein, however, the term “geometrically ordered multicrystalline silicon” (hereinafter abbreviated as “geometric multicrystalline silicon”) refers to crystalline silicon, according to embodiments of the present invention, having a geometrically ordered centimeter scale grain size distribution, with multiple ordered crystals located within a body of multi-crystalline silicon. For example, in geometric multi-crystalline silicon, the grains are typically an average of about 0.5 cm to about 5 cm in size, and grain orientation within a body of geometric multi-crystalline silicon is controlled according to predetermined orientations.
Further, as used herein, the term “polycrystalline silicon” refers to crystalline silicon with micrometer scale grain size and multiple grain orientations located within a given body of crystalline silicon. For example, the grains are typically an average of about submicron to about micron in size (e.g., individual grains are not visible to the naked eye), and grain orientation distributed randomly throughout. In the procedure described above, the ingots or blocks are cut into thin substrates, also referred to as wafers, by known slicing or sawing methods. These wafers may then be processed into photovoltaic cells.
Monocrystalline silicon for use in the manufacture of photovoltaic cells is generally produced by the CZ or FZ methods, both being processes in which a cylindrically shaped boule of crystalline silicon is produced. For a CZ process, a seed crystal is touched to a pool of molten silicon and the boule is slowly pulled out of the pool while heat is extracted through the solid part of the boule. As used herein, the term “seed crystal” refers to a piece of crystalline material that is brought in contact with liquid silicon such that, during solidification, the liquid silicon adapts to the crystallinity of the seed. For a FZ process, solid material is fed through a melting zone, melted upon entry into one side of the melting zone, and re-solidified on the other side of the melting zone, generally by contacting a seed crystal.
Recently, a new technique for producing monocrystalline or geometric multicrystalline material in a casting station has been invented, as disclosed in U.S. patent application Ser. Nos. 11/624,365 and 11/624,411 and published as U.S. Patent Application Publication Nos. 20070169684A1 and 20070169685A1, filed Jan. 18, 2007. Casting processes for preparing multicrystalline silicon ingots are known in the art of photovoltaic technology. Briefly, in such processes, molten silicon is contained in a crucible, such as a quartz crucible, and is cooled in a controlled manner to permit the crystallization of the silicon contained therein. The block of cast crystalline silicon that results is generally cut into bricks having a cross-section that is the same as or close to the size of the wafer to be used for manufacturing a photovoltaic cell, and the bricks are sawn or otherwise cut into such wafers. Multi-crystalline silicon produced in such manner is an agglomeration of crystal grains where, within the wafers made therefrom, the orientation of the grains relative to one another is effectively random. Monocrystalline or geometric multicrystalline silicon has specifically chosen grain orientations and (in the latter case) grain boundaries, and can be formed by the new casting techniques disclosed in the above-mentioned patent applications by bringing liquid silicon in contact with a large seed layer that remains partially solid during the process and through which heat is extracted during solidification. As used herein, the term ‘seed layer’ refers to a crystal or group of crystals with desired crystal orientations that form a continuous layer. They can be made to conform with one side of a crucible for casting purposes.
In order to produce the best quality cast ingots, several conditions should be met. Firstly, as much of the ingot as possible have the desired crystallinity. If the ingot is intended to be monocrystalline, then the entire usable portion of the ingot should be monocrystalline, and likewise for geometric multicrystalline material. Secondly, the silicon should contain as few imperfections as possible. Imperfections can include individual impurities, agglomerates of impurities, intrinsic lattice defects and structural defects in the silicon lattice, such as dislocations and stacking faults. Many of these imperfections can cause a fast recombination of electrical charge carriers in a functioning photovoltaic cell made from crystalline silicon. This can cause a decrease in the efficiency of the cell.
Many years of development have resulted in a minimal amount of imperfections in well-grown CZ and FZ silicon. Dislocation free single crystals can be achieved by first growing a thin neck where all dislocations incorporated at the seed are allowed to grow out. The incorporation of inclusions and secondary phases (for example silicon nitride, silicon oxide or silicon carbide particles) is avoided by maintaining a counter-rotation of the seed crystal relative to the melt. Oxygen incorporation can be minimized using FZ or Magnetic CZ techniques as is known in the industry. Metallic impurities are generally minimized by being left in the potscrap or the tang end after the boule is brought to an end.